+1 Gravity works fine locally to within our limits of precision but the rotation of galaxies and clusters of galaxies need dark matter or those observations falsify the gravitation theory. Since dark matter has not been found gravity doesn't work fine on a large scale.
– Frank HubenyMar 8 '18 at 13:35

Ok. But why do we want to achieve unified theory of everything ? Dark matter only suggests existence of new kind of force. I think it is wrong to expect that one seed can lead to growth of apple and oranges on the same tree.
– Dheeraj VermaMar 8 '18 at 13:50

2

I hope someone will answer your main question, Dheeraj. My answer is only partial. A belief that we can find a unified theory of everything implies a belief that (1) the universe is intelligible to us and (2) it is ultimately deterministic requiring no agents of any sort. I think the first is true, but the second is false.
– Frank HubenyMar 8 '18 at 13:58

Because almost all knowledge is an integration of atomic thoughts and theories. If two theories cannot be integrated then one must be wrong so integrating them is part of testing them, and when they are integrated we will know more about both.
– PeterJMar 9 '18 at 10:30

3 Answers
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At the present time, we do know not 2, but 4 kinds of physical interaction forces:

Gravity

Electromagnetism

Weak interaction (explaining phenomena from radioactivity)

Strong interaction (explaining phenomena from particle physics)

The latter three kinds of interaction have been successfully unified within the standard model by a common type of mathematical formalism (gauge theory). The same formalism could not be extended to include gravity.

Therefore we now have a dichotomy between on one hand quantum field theory which unifies the latter three kinds of interaction. And on the other hand the theory of relativity which covers gravity. Both theories follow quite a distinct paradigm.

During its early phase our cosmos was a world with extremely high energy enclosed in an extremely small domain of space. There was the very active interaction of particles and radiation in a highly curved spacetime. These phenomena cannot be explained by applying the theory of relativity and quantum field theory in separation.
We need a unified theory, a theory of quantum gravity. Such theory does not exist up to now. The two main candidates for a theory of quantum gravity are loop quantum gravity and string theory.

Besides a quantum theory of gravity, etc, as mentioned by @JoWehler, consider the first half-or-so of the 1800's, where we had more-or-less separate theories of electricty and magnetism. That worked fine for inventing the telegraph and similar stuff, and maybe you might have thought "why would we want to unify them [electricty&magnetism]?"

But then Maxwell came along in the 1860's and did exactly that (adding a new "displacement current" equation to conserve charge). And then, voila, strange and wonderfgul things emerged that had never before been imagined. For example, by combining the equations for electricity and magnetism, an entirely new equation emerged, predicting the existence of electromagnetic waves. Moreover, it predicted the speed of those waves, sqrt(1/mu*epsilon), simply from measurable electronic properties of material (including the vacuum). And that turned out to be exactly the same as the already-measured speed of light, whereby light was finally understood to be an electromagnetic wave. That would never have happened without unifying electricity and magnetism, and we'd never have invented radio, television, etc.

Who knows what strange and wonderful new things will emerge from further unification? Quantum electrodynamics has already succeeded in unifying Maxwell's electromagnetic theory and quantum mechanics. And that's indeed revealed wonderful new insights, although nobody's invented a "stargate" or any such thing as an outgrowth of that. Not yet, anyway. And pretty much ditto for the standard model (aka quarks and such stuff), leaving gravity the only not-yet-unified fundamental force. Whether or not unifying them all opens the door to a vast new technological regime of astounding gadgets remains to be seen. But astounding new insights are an absolute certainty.